1. Field of the Invention
This invention relates to permanent magnets and more particularly, to polycrystalline Mn-Al-C ally magnets of high performance suitable for use in multipolar magnetization. Also, it relates to a method for making the magnets of the just-mentioned type.
2. Description of the Prior Art
Mn-Al-C alloy magnets have mainly the ferromagnetic face-centered tetragonal phase structure (.tau. phase L1o type superstructure) and comprises carbon as their essential component element. The Mn-Al-C alloy magnets include those magnets of the ternary alloys free of any additive elements except for inevitable impurities and quaternary or multicomponent alloys which contain small amounts of additive elements. By the term "Mn-Al-C alloy magnet" used herein are meant magnets of the alloys including quaternary or multicomponent alloys as well as the ternary alloys.
Known methods of making Mn-Al-C alloy magnets include, aside from those methods using casting and heat treatments, a method which comprises a warm plastic working process such as warm extrusion. The latter method is known as a method of making an anisotropic magnet which has excellent properties such as high magnetic characteristics, mechanical strength and machinability.
On the other hand, Mn-Al-C alloy magnets for multipolar magnetization can be made by several methods including a method using isotropic magnets or compressive working, and a method in which a uniaxially anisotropic polycrystalline Mn-Al-C alloy magnet obtained by a known technique such as warm extrusion is subjected to warm free compressive working in a direction of easy magnetization, i.e. a compound working method.
However, the compressive working method involves the drawbacks that although high magnetic characteristics are obtained in radial directions, a relatively high reduction rate is necessary, non-uniform deformation may take place, and occurrence of a dead zone is unavoidable. According to the compound working method, there can be obtained magnets which exhibit high magnetic characteristics in all the directions within a plane including radial and tangential directions in small compressive strains. The magnets obtained by the compound working method have such a structure that the direction of easy magnetization is parallel to a specific plane, and they are magnetically isotropic within the plane and are anisotropic within a plane including a perpendicular with respect to the first-mentioned plane and a straight line parallel to the first-mentioned plane. These magnets are hereinafter referred to as plane-anisotropic permanent magnet.
Magnets for multipolar magnetization are generally in the form of a hollow cylinder and are magnetized as particularly shown in FIGS. 1 through 3 in which magnetic paths are indicated by broken lines. FIG. 1 is a schematic diagram of magnetic paths in a magnet body in case where a hollow cylindrical magnet undergoes multipolar magnetization in radial directions. FIG. 2 shows a case where a hollow cylindrical magnet is multipolarly magnetized around the outer circumferential surface and FIG. 3 shows a case of multipolar magnetization around the inner circumferential surface of a cylindrical magnet. The magnetization shown in FIG. 1 is called radial magnetization throughout the specification. Similarly, those magnetizations shown in FIGS. 2 and 3 are called outer lateral or circumferential magnetization and inner lateral or circumferential magnetization. In FIG. 2, radial directions are indicated by r and a tangential direction with respect to one radial direction is indicated by .theta..
As shown in FIG. 1, with the radial magnetization, the magnetic paths substantially run along the radial directions and thus the structure of the above-mentioned plane-anisotropic permanent magnet may not necessarily be proper. On the other hand, according to the compressive working technique, high magnetic characteristics along radial directions can be obtained. However, as described before, this working technique involves the problems that a relatively high reduction rate is required, non-uniform deformation may occur and occurrence of a dead zone is unavoidable.
Plane-anisotropic permanent magnets are magnets of versatile utility which exhibit excellent magnetic characteristics when magnetized in the manners shown in FIGS. 1 through 3. In this connection, however, if consideration is given, for example, to the outer circumferential magnetization, the plane-anisotropic permanent magnet has not necessarily a favorable anisotropic structure at its outer or inner circumferential portion. With regard to the outer circumferential portion of a magnet body, it should favorably have higher magnetic charcteristics in radial directions than in tangential directions. On the other hand, so far as an inner circumferential portion is concerned, an anisotropic structure having higher magnetic characteristics in tangential directions than in radial directions is more suitable for outer circumferential magnetization. It will be noted that the outer circumferential portion of a magnet body means a portion where magnetic paths run substantially along radial directions and the inner circumferential portion means a portion where the magnetic paths run substantially along tangential directions, as particularly seen in FIG. 2.